Soldering is a widely used method of joining two metals together by means of a third low-melting metal layer. Effective soldering relies on the solder metal being liquified and wetting the metals to be joined and then being cooled to resolidify the solder. A flux is normally used in this process to clean the metal surfaces to be joined, i.e., to remove surface layers of oxide, sulfide, carbonate, grease and the like formed by exposure of the metals to the atmosphere. The flux also displaces adsorbed gases on the metallic surfaces which interfere with good wetting by the molten solder.
In the most common use of solder, the solder or solder-flux combination is melted by a source of heat such as an iron. When small parts are to be joined, or small devices affixed to a metal board in a predetermined position, particularly in the electronics industry, a small directed heat source is required. Recently carbon dioxide laser beams have come into use for such applications. Lasers promote rapid solder fusion and cooling, thus avoiding formation of intermetallic compounds during soldering. They can also be finely controlled for automated assemblies and they provide a non-contacting heat source. However, the carbon dioxide lasers used heretofore are expensive and require relatively large amounts of power to activate them. However, when other lasers, such as Nd:YAG lasers, were tried as heat sources, it was found that the metallurgical properties of the bond were less than desirable and that unexpectedly high power was required to melt the solder-flux combination. Further, the operation was less efficient than expected due to inadequate heating of the solder by the laser.